The ontogeny of B-lymphocyte function was studied in a cell transfer system. B cells from neonatal or fetal animals were transferred to lethally irradiated syngeneic recipients together with 10 to the 8th power adult thymus cells. It was found that the B-cell pouplation acquired the capacity to produce an adult-like heterogeneous response between seven and ten days after birth. It was shown that this maturation event required or was facilitated by thymus cells. Evidence was produced that maturation of the thymus to be capable of facilitating B cell maturation represented an important control regulating B-cell ontogeny.